1. Field of the Invention
This invention relates to charge-coupled devices (CCD) and, more particularly, to techniques for improving the performance of such devices in the presence of ionizing radiation, e.g., x-rays, gamma rays, and energetic charged particles.
2. Prior Art
The concept of a Schottky-barrier infra-red (IR) photon detector, which utilizes internal photo emission of "hot" carriers (holes), was first proposed by Shepherd and Yang in 1973. The Schottky barrier approach to monolithic IR CCDs is based on internal photo emission from metal-semiconductor Schottky barrier arrays on a silicon substrate. Hot carriers (holes) are generated in the metal by absorption of photons with wavelengths greater than cutoff that penetrate through the semiconductor. Those hot carriers, or holes, with proper energy and momentum transit the contact barrier potential and are emitted into the semiconductor, where they neutralize fixed charges at the edge of the depletion region induced by application of a reverse-bias voltage. Signal readout may be accomplished by a MOS transfer gate conversion of majority carriers to minority carriers required by the Si CCD. As a result of the photo-detection in the metal film, the Schottky barrier devices have highly uniform responsivity across an array but, while they have relatively low photo emissive yield in comparison to other CCDs, experience has shown that adequate IR imagery can be achieved.
A Schottky barrier detector is a photon detector, which utilizes the internal photo emission of "hot" carriers (holes) from metal or metal silicide electrodes into silicon for the detection of radiation. For the hole emission process, a back-illuminated Schottky barrier detector operates in the spectral window between the band gap of silicon (Eg) and the so-called Schottky barrier height (.psi.). Photons with energies between the band gap and the barrier height--that is, Eg&lt;h.nu.&lt;.psi.--will be transmitted through the silicon substrate and absorbed in the metal electrode of the junction.
A problem arises when ionizing radiation passes through the silicon substrate because a penetrating energetic charged particle generates electron-hole pairs along its path by ionization of atoms in the substrate. The problem is that the ionizing radiation passing through the silicon substrate generates electrical charge carriers that are also collected by the active charge collection area. The charge generated by the ionizing radiation is large enough to mask the image signal and can saturate the active area of a pixel. Ionizing radiation can render CCD cameras useless as imaging devices and is a significant problem for CCD cameras in space and nuclear weapon radiation environments.
A need exists for a technique for reducing the effect of the electrical charge that is generated by ionizing radiation in the silicon substrate of a CCD detector.